1. Field of the Invention
The present invention generally relates to non-linear optics, and more particularly to quenching or suppression of optical breakdown induced by propagation of a strong laser field through a non-linear medium in a cell used for non-linear optics.
2. Description of the Related Art
Optical breakdown (OBD) is the catastrophic evolution of damage inflicted in a transparent medium by a strong laser field, resulting in electron avalanche and plasma formation. The process begins with a small number of free electrons which naturally occur in the medium, or are generated by laser-induced (multiphoton) ionization. Electron avalanche can develop if the electrons are able to gain energy from the laser field, since they can then attain enough energy to ionize an atom in collision, and repetitions of the process will lead to a rapid multiplication of electrons. Cascade ionization or electron avalanche follows, with the resultant formation of a plasma. As soon as the level of ionization becomes appreciable, the incoming light can be readily absorbed by free electrons via free-free transitions in the field of ions. This causes intense heating of the electron plasma and a consequent rapid hydrodynamic expansion of the plasma in the form of a shock wave. The final result is the appearance of a spark in the medium. A general treatise on OBD is found in "The Principles of Non-Linear Optics, Chapter 27, John Wiley and Sons, New York, 1984.
OBD is a serious problem in applications of non-linear optics to high-energy and medium-energy laser systems. For example, OBD competes strongly with stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) under a number of conditions. SBS and SRS are both important mechanisms for optical phase conjugation, beam combining, beam clean-up, wavelength shifting, and other applications.
Electric fields have previously been used to clean up exhaust from factory chimneys in what is known as a Cottrell process. Particles of soot are ionized in the corona of an applied electric field. The soot is then drawn out of the chimney by an applied electric field and deposited on some electrodes. This process, developed in the early 1900's, was designed to reduce the amount of soot in chimney exhaust, and is discussed in a textbook entitled "General Inorganic Chemistry", by J. L. Maynard et al, Van Nostrand Co., New York 1942, pp. 775-778. By contrast, the present invention could use either optical or electrical fields to ionize contaminants, and furthermore, the applied electric field need not necessarily remove the contaminants from the transparent medium, but need to only remove it from the region of the overlap between the optical beam and the medium. In the present invention, a nominally-pure medium is made ultra-pure in order to affect its optical breakdown properties.
Another related art known as "ion plating" was developed and patented by D. M. Mattox at the Sandia National Laboratories, and is described in a textbook entitled "Ion Plating Technology Developments and Applications", by N. Ahmed, John Wiley & Sons, 1987, pp. 1-12. This is a means for depositing films on metallic substrates by using the substrates as electrodes in a high-vacuum environment. In this system, atoms are electrically ionized and electrically drawn to the metallic substrates where they stick, thus coating the substrate uniformly. By contrast, in the present invention, atoms or groups of atoms can be ionized either optically or electrically. The purpose of the present invention is not to obtain a good coating, but to purify the non-linear optical medium.
Finally, getter-ion pumps are high-vacuum pumps which "clean-up" inert gases by electrically ionizing atoms or groups of atoms and drawing the positive ions to the wall. This process is described in a textbook entitled "Vacuum Technology", by A. Roth, North-Holland, New York, 1976, pp. 227-237. Again, in the present invention, contaminants can be ionized either optically or electrically. Furthermore, the present invention has been demonstrated to work under high-pressure conditions. Finally, the end result of the present invention is to increase the threshold for OBD; getter-ion pumps are designed to obtain ultra-high vacuum.
All three examples of prior art described above use an electric field to cause electrical breakdown. By contrast, in the present invention, the applied electric field quenches or suppresses optical breakdown.
The problem posed by OBD is particularly severe for SBS phase conjugation applications involving pulse lengths shorter than the Brillouin lifetime. In this case, relatively high peak powers are needed to obtain above-threshold operation of SBS; these high peak powers approach and surpass the OBD threshold of many non-linear materials. Laser pulse formats involving trains of short pulses generated by radio frequency free electron lasers (rf FELs) or mode-locked solid state lasers are particularly prone to OBD problems since the inter-pulse time interval of these systems is short relative to the amount of time it takes for electrons to diffuse out of the focal volume.